1. Field of the Invention
The present invention relates generally to a stereophonic demodulation system and particularly to a novel stereophonic demodulation system for demodulating a stereophonic signal free of any accompanying useless signal.
2. Description of the Prior Art
In prior art stereophonic demodulator apparatus, when a stereophonic composite signal of the type used in frequency modulation (FM) four-channel broadcasting is demodulated, a switching signal having a frequency of f.sub.1, for example, 38 KHz is utilized. This switching signal contains a third harmonic component having a frequency of 114 KHz that causes various kinds of characteristic deterioration of the desired signal. Mathematically, the stereophonic composite signal F(t) used in four-channel broadcasting can be expressed by equation (1), as follows: EQU F(t) = a + b sin .omega.t + c cos .omega.t + d sin 2.omega.t + p sin (.omega./2) t (1)
where: EQU .omega. = 2.pi.f.sub.1, EQU f.sub.1 = 38 KHz, EQU a = L.sub.F + L.sub.R + R.sub.F + R.sub.R, EQU b = L.sub.F + L.sub.R - R.sub.F - R.sub.R, EQU c = L.sub.F - L.sub.R + R.sub.F - R.sub.R, EQU d = L.sub.F - L.sub.R - R.sub.F + R.sub.R,
l.sub.f is the amplitude of the left-front signal,
L.sub.r is the amplitude of the left-rear audio signal, PA1 R.sub.f is the amplitude of the right-front audio signal, PA1 R.sub.r is the amplitude of the right-rear audio signal, PA1 And p sin (.omega./2) t represents a pilot signal.
The signal components of the stereophonic composite signal F(t), that is, the main signal a, the first sub-channel signal b sin .omega.t, the second sub-channel signal c cos.omega.t, and the third sub-channel signal d sin 2.omega.t, occupy different parts of the frequency spectrum. So-called triangular noise signals D, E and F in frequency bands corresponding to parts of the frequency spectrum occupied by the information signals are detected by an FM detector that is supposed to demodulate only the stereophonic signal. In addition to these triangular noise signals, there is another triangular noise signal G at a higher frequency band: the third harmonic component band of the 38 KHz carrier (114 .+-. 15 KHz). The amplitude of this triangular noise G is very high. In addition, a harmonic component of a stereophonic composite signal having a frequency less than about 65 KHz is included in the third harmonic component band of the demodulated signal, so that the phenomena of deterioration in signal-to-noise ratio (S/N), distortion factor, adjacent channel interference eliminating characteristics, and the like are produced within the audio frequency range. In order to avoid these undesirable effects, the useless signal component in the third harmonic band of 114 KHz may be removed by a low pass filter or band pass filter. However, these filters may have a bad effect on the third sub-carrier signal d sin 2.omega.t having a carrier frequency of 76 KHz contained in the stereophonic composite signal F(t). The phase characteristics of the third sub-channel signal d sin 2.omega.t are likely to be deteriorated and thus cause deterioration of separation characteristics. As a result, it is difficult to improve the deterioration in the S/N ratio, the distortion factor, adjacent channel interference eliminating characteristics, and the like, which are caused by the effect of third harmonic components of a demodulated signal.
Further, it is noticed that when a normal two-channel stereophonic composite signal is demodulated, if an adjacent broadcasting signal exists near the frequency to be received, the demodulated audio frequency band is badly affected by the third harmonics of 38 KHz.